The present invention pertains to field emission display (xe2x80x9cFEDxe2x80x9d) devices. More particularly, the invention relates to a system for controlling brightness of a FED.
Until recently, the cathode ray tube (xe2x80x9cCRTxe2x80x9d) has been the primary device for displaying information. While having sufficient display characteristics with respect to color, brightness, contrast and resolution, CRTs are relatively bulky and power hungry. These failings, in view of the advent of portable laptop computers, has intensified demand for a display technology which is lightweight, compact, and power efficient.
One available technology is the flat panel display, and more particularly, the liquid crystal display (xe2x80x9cLCDxe2x80x9d). LCDs are currently used for laptop computers. However, LCDs provide poor contrast in comparison to CRT technology. Further, LCDs offer only a limited angular display range. Moreover, color LCD devices consume power at rates incompatible with extended battery operation. In addition, a color LCD type screen tends to be far more costly than an equivalent CRT.
In light of these shortcomings, there have been several developments recently in thin film, field emission display (xe2x80x9cFEDxe2x80x9d) technology. In U.S. Pat. No. 5,210,472, commonly assigned with the present invention, and incorporated herein by reference, a FED design is disclosed which utilizes a matrix-addressable array of pointed, thin-film, cold cathode emitters in combination with a conductive, transparent screen having a conductive coating which is in turn, coated with a cathodoluminescent material. An extraction grid having a plurality of openings aligned with respective emitters is positioned between the emitters and the screen. The screen is biased at a relatively high voltage on the order of 80V to 1KV. When the voltage of the extraction grid is sufficiently higher than the voltage of the emitters, electrons are emitted from the underlying emitter and are attracted to the conductive screen. When the electrons strike the cathodoluminescent material, light is emitted at the point of impact. The intensity of the emitted light is proportional to the rate at which electrons are emitted which is, in turn, proportional to the voltage differential between the extraction grid and emitter. The FED incorporates a column signal to activate a single column extraction grid, while a row signal activates a row of emitters. At the intersection of both an activated column and an activated row, a grid-to-emitter voltage differential exists sufficient to induce electron emission. Extensive research has recently made the manufacture of an inexpensive, low power, high resolution, high contrast, full color FED a more feasible alternative to LCDs.
In order to achieve the advantages of this technology, as in the performance of LCDs, FED devices require a brightness control scheme. Several techniques have been proposed to control the brightness and gray scale range. For example, U.S. Pat. No. 5,103,144 to Dunham and U.S. Pat. No. 5,103,145 to Doran, both incorporated herein by reference, teach methods for controlling the brightness and luminance of flat panel displays. However, a need remains for a brightness control scheme that requires less power and is simpler to manufacture. Further, a need exists for a brightness control scheme requiring less circuitry and thus less surface area on a silicon die.
Accordingly, a flat panel display of the present invention, includes an emitter current control circuit that controls an emitter set in a FED. The current control circuit converts an analog input to a control signal to control the rate at which electrons are emitted by the emitter set, where the rate of electron emission corresponds to the analog input signal""s amplitude.
In one embodiment of the present invention, a gray scale generator adjusts the gray scale range of the FED to provide contrast to the FED.
In another embodiment of the invention, an optical sensor senses ambient light surrounding the flat panel display and produces an electrical signal in response thereto. The control circuit receives the electrical signal and modifies the control signal in response.
In another embodiment of the invention, the current control circuit includes a parasitic capacitance coupled to a control line, such as a column line, by a pass transistor. The pass transistor selectively couples a control voltage from the control line to the parasitic capacitance to charge the parasitic capacitance. The pass transistor then turns OFF to isolate the parasitic capacitance and trap the control voltage on the parasitic capacitance. The trapped control voltage drives the gate of an NMOS transistor coupled between the emitter set and ground. In response to the control voltage, the NMOS transistor passes current so that the emitter set emits electrons, thereby illuminating a pixel of the display.
Other advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawings attached hereto.